1. Field of the Invention
The present invention relates to a method for controlling an exhaust system of a vehicle by regulating area of a flow passage in an exhaust pipe depending upon operational condition of the vehicle. More particularly, the present invention relates to a method for assigning an optimum control pattern to an on-off valve disposed in an exhaust system for discharging exhaust gas from a driving engine of a vehicle depending upon running condition of the vehicle, thus increasing the engine output and/or reducing a noisy sound from the engine to a desired degree.
2. Description of Related Art
The volume of exhaust gas discharged from a driving engine of a vehicle will constantly vary depending upon an engine speed and/or running condition of the vehicle. If an engine is provided with an exhaust system the construction of which remains unchanged regardless of varying running condition of the vehicle, the engine would represent insufficient efficiency and/or a noisy sound resulting from exhaust gas from the engine would become louder.
That is, if the engine is provided with an exhaust system which is effective enough to damp noisy sound from the engine during a relatively small quantity of exhaust gas is discharged from the engine, such exhaust system may cause a large resistance to the exhaust gas when the quantity of exhaust gas becomes greater, thus restricting the engine from producing higher output. On the contrary, and if the engine is provided with an exhaust system which does not cause a very high resistance to a flow of exhaust gas during a large quantity of exhaust gas is discharged from the engine, such an exhaust system would be incapable of reducing or damping a noisy sound to a desired level.
Under the circumstances, an exhaust system of a variable construction which is designed to vary the area of an exhaust gas flow passage depending upon running condition of a vehicle, such as shown in Japanese Utility Model Public Disclosure No. 62-54210, has been proposed and is employed in some vehicle engines.
Such an exhaust system of variable construction includes, as shown in FIG. 7, a pair of exhaust pipes 1 and 2 each communicating with an exhaust port of an engine and arranged in parallel with each other, an on-off valve 3 disposed at an intermediate portion of one exhaust pipe 1, and a controller 4 for controlling the on-off valve 3 depending running condition of a vehicle.
In FIG. 7, reference numeral 5 designates an actuator which actuates to open or close the on-off valve 3 in accordance with a signal from the controller 4 and numeral 6 a silencer which receives flow of exhaust gas from the engine for reducingly damping components of a noisy sound accompanying the exhaust gas. The pair of exhaust pipes 1 and 2 are incorporated in the silencer 6 so as to constitute outlet conduits thereof.
A signal indicative of an accelerator opening and a signal indicative of an engine speed are input to the controller 4. The controller 4 controls an open-close operation of the on-off valve 3 in accordance with the above two signals, as shown in FIG. 8.
Specifically, the on-off valve 3 is opened when the engine speed is very high (for example, no less than 4,600 r.p.m), or when the engine speed is relatively high (for example no less than 2,500 r.p.m.) and at the same time the accelerator opening is relatively large (for example, no less than 40%), whereas the on-off valve 3 is closed in any other cases.
As a result, when a large quantity of exhaust gas is discharged from the engine, the area of a flow passage of exhaust gas becomes greater so as to prevent occurrence of a large amount of resistance against the exhaust gas flow, whereby maintaining sufficient amount of engine output. On the contrary, and when a relatively small quantity of exhaust gas is discharged from the engine, the area of the flow passage of exhaust gas is restricted so as to reducingly damp noisy sound to a desired degree.
It has been found, however, that the prior art exhaust system of variable construction for a vehicle having the construction and function as shown above is incapable of satisfactorily functioning when the engine is rapidly accelerated.
That is to say, the controller 4 is designed to detect the engine speed and the accelerator opening so as to open the on-off valve 3 when the engine speed is instantaneously increased in response to a rapid acceleration motion. The on-off valve 3, however, will not be opened until after the engine speed has reached 2,500 r.p.m. even if the accelerator is fully opened. Thus, it is unavoidable for such a prior art exhaust system to represent a slight time-lag until the on-off valve 3 actually opens.
During such a time-lag, the volume of exhaust gas discharged from the engine continuingly increases. The area of the flow passage of exhaust gas however remains narrow, thus forming a considerable amount of resistance to the exhaust gas flow. Accordingly, the amount of back pressure within the engine increases, thereby restricting the engine from producing higher output.
If, on the other hand, it is intended to minimize the above time lag by setting a predetermined level of the engine speed to commence the opening operation of the on-off valve 3 at a lower point, it would be possible to obtain an increased output of the engine upon rapid acceleration operation. This may cause, however, a change in the damping characteristics of the silencer when the engine operates at a lower speed slightly higher than such a predetermined level, thus resulting in resonance noise due to occurrence of resonant vibration of the vehicle chamber by means of discharge sound and vibration of the exhaust system, whereby causing insufficient silencer effect at a lower engine speed.